Aluminum nitride (AlN) is a refractory material melting at 2400.degree. C., which exhibits several unique chemical and physical properties, e.g., it has a density of 3.26 g/cm.sup.3, a Young's modulus of 280 GPa, a flexural strength of 400 MPa and a Knoop hardness of 1200 kg/mm.sup.2. AlN is very stable in the presence of molten metals and therefore can be used, for example, for making crucibles to hold molten metal.
Aluminum nitride is also an electrical insulator with a bandgap of 6.2 electron volts, which makes it an attractive alternative substrate material to replace alumina and beryllia in electronic packaging. The thermal expansion coefficient of AlN is nearly identical to that of silicon. This is an important property in high power applications where thermal distortion can occur between a silicon chip and the substrate due to a mismatch in the coefficients of thermal expansion of the two materials. The thermal conductivity of aluminum nitride is nearly ten times higher than alumina and approximately equal to that of beryllia. Unlike beryllia, aluminum nitride is not restricted by processing constraints because of its toxicity.
There is currently a great deal of interest in polymer precursor materials that can be pyrolyzed to yield ceramic materials, including aluminum nitride. Aluminum-nitrogen polymers containing no alkyl substitution on the aluminum or nitrogen atoms are described in U.S. Pa. No. 4,767,607, in which thermolysis of a mixture of aluminum chloride and hexamethyldisilazane results in formation of a polymer with the repeating unit --(C1)Al-N(H)].sub.n. Pyrolysis of the polymer in ammonia or under vacuum yields crystalline AlN. An infusible polymeric aluminum amidimide --(NH.sub.2)Al-N(H)].sub.n that can be pyrolyzed to form AlN is described by L. Maya, Adv. Ceram. Mat., 1986, 1, 150-153.
Polymers having the repeating unit --(R)Al-N(H)].sub.n are disclosed in U.S. Pat. No. 4,696,968 and European Patent Application 259,164. Fibers can be melt spun from the thermoplastic precursor and pyrolyzed to AlN. L. V. Interrante et. al., Inorganic Chem., 1989, 28, 252-257 and Mater. Res. Soc. Symp. Proc., 1986, 73, 359-366 reported the formation of volatile crystalline precursors that can be sublimed under vacuum. A two step pyrolysis of these precursors in ammonia resulted first in an insoluble aluminum imide polymer of the form --(R)Al-N(H)].sub.n and ultimately AlN containing less than 0.5% residual carbon and oxygen. U.S. Pat. NO. 4,783,430 discloses the formation of --(CH.sub.3)Al-N(H)].sub.n, which can be pyrolyzed under helium, argon or vacuum to form hexagonal AlN.
Polymers having the repeating unit --(H)Al-N(R)].sub.n are disclosed in U.S. Pat. No. 3,505,246 and are formed by the reaction of the alane adduct H.sub.3 A1.fwdarw.N(C.sub.2 H.sub.5).sub.3 with a reagent such as acetonitrile. U.S. Pat. No. 4,687,657 discloses the preparation of a poly-N-alkyliminoalane that can be pyrolyzed in argon or under vacuum to form AlN.
Reacting an organic nitrile with diisobutylaluminum hydride produced organoaluminum imines having the formula RCH=NAi(i-C.sub.4 H.sub.9).sub.2, which were not isolated (L. I. Zakharkin and I. M. Khorlina, Bull, Acad. Sci. USSR, Engl. Transl., 1959, 523-524 and Proc. Acad. Sci. USSR, 1957, 112,879). A gas containing 85% isobutene and polymers having the repeating unit --Al-N(R)].sub.n were produced on heating the organoaluminum imine to 220.degree. to 240.degree. C. During the formation of the polymer, aluminum alkyl groups of the organoaluminum imine are eliminated as isobutene, and aluminum-nitrogen bonds are formed.
European Patent Application 331,448 discloses that AlN can be deposited on a substrate by heating the substrate and contacting it with the vapor of an aluminum-nitrogen compound having the formula CH.sub.3 (R.sup.1)Al-N(R.sup.2)(C.sub.3 H.sub.7), where R' is alkyl and R.sup.2 is H, alkyl or aryl. A polymer of this compound is claimed, but the structure of the polymer is not disclosed.